A comparative trial of amitriptyline and phenelzine is in assess pharmacologic and therapeutic effects of these two antidepressant drugs in the treatment of outpatients with depression. Patients are comprehensively evaluated by means of a structured depression interview (SDI), a self-rating symptom checklist (SCL-90), and other global and side effects scales. There is emphasis on identification of patient subgroups with depressive neuroses, atypical depression, etc., who respond preferentially to one type of antidepressant drug. Plasma drug concentrations and platelet MAO inhibition are measured every two weeks and compared to clinical improvement and side effects. Differential antidepressant drug response will be assessed, and predictive functions of clinical utility developed.